This invention relates to the heating of a cooking utensil which is inductively coupled to a flat, pancake-shaped coil excited by an ultrasonic power oscillator.
More particularly, this invention relates to a central power oscillator for simultaneously exciting multiple induction cooking coils.
Many prior art induction cooking units having multiple induction cooking coils adapted and arranged to function as the burners of a domestic kitchen range, utilize a separate power oscillator for each induction coil. In such a system, power to a particular coil can be controlled by varying the operating parameters of its respective oscillator without affecting a power change in the other induction coils. For example, the operating voltage of the oscillator exciting the induction coil could be varied to control power as disclosed in U.S. Pat. No. 3,806,688 which is assigned to the assignee of this invention. Power in an induction cooking coil can also be controlled by varying the frequency of its associated power oscillator, especially where the coil is excited at a resonant or near resonant frequency. U.S. Pat. No. 3,781,505 discloses an induction cooking apparatus utilizing oscillator frequency variance for power control.
While an induction cooking apparatus utilizing separate oscillators for each induction cooking coil may yield suitable performance, the wide-spread commercialization of such an apparatus has not materialized due for the most part to prohibitive cost. An induction cooking apparatus utilizing a central oscillator powering, for example, four individual induction coils for simultaneously heating four cooking utensils would apparently be a solution to the cost problem associated with multiple oscillator induction cooking devices. However, the problem of controlling the individual coils without having a change in one coil affect the power outputs of the other coils has prevented the practical application of a central oscillator approach. Obviously, the operating parameters of a central oscillator could not be changed without affecting all of the induction heating coils. Therefore, it is necessary to devise a control means for varying the power to each induction coil without changing the operating parameters of the central oscillator.
In addition to the control need noted above, the provision of a common oscillator capable of powering four induction cooking coils would most likely require excessive semiconductor paralleling in order to handle the required power. Such semiconductor paralleling was considered undesirable especially from a reliability standpoint.